Add continuous integration builder

Building tailscale-debug.apk also runs unit tests.

Signed-off-by: Denton Gentry <dgentry@tailscale.com>

.github/workflows/build.yml

@@ -0,0 +1,23 @@
+name: Build Debug APK
+
+on:
+  push:
+    branches:
+      - main
+  pull_request:
+    branches:
+      - '*'
+
+jobs:
+  build:
+    runs-on: ubuntu-latest
+
+    if: "!contains(github.event.head_commit.message, '[ci skip]')"
+
+    steps:
+
+    - name: Check out code
+      uses: actions/checkout@v3
+
+    - name: Build APK
+      run: make tailscale-debug.apk